Fuel versus payload to Great Barrier
You are planning a PA-28-181 from Ardmore to Great Barrier Island (Claris), 2.2 hours still-air at 38 L/h. The aircraft has a basic empty weight of 750 kg and MTOW 1,157 kg; usable fuel is 182 L at 0.72 kg/L. Payload is pilot 85 kg plus three passengers at 75 kg each. Fuel planning uses 4 L for taxi, 38 L/h cruise, and a 45-minute fixed reserve. The forecast is for a 25 kt headwind at cruise, giving 2.7 hours. The refueller can deliver any quantity, but the station manager wants to know if you can still take all four people. There is no fuel at Claris; the alternate is Whangarei, 45 minutes away. You have 135 L in the tanks from a previous flight and the bowser is available.
1. Calculate the maximum fuel that can be carried with all four occupants and compare it with the tank capacity.
Read the worked answer
Payload = 85 + 3 × 75 = 310 kg. BEW + payload = 750 + 310 = 1,060 kg. Allowable fuel mass = 1,157 − 1,060 = 97 kg. At 0.72 kg/L: 97 / 0.72 = 134.7 L.
With all four on board you cannot fill the 182 L tanks; the weight-limited maximum is about 135 L. The aircraft would be at MTOW with that fuel, so any extra baggage or a heavier passenger immediately reduces the fuel that can be carried.
2. Calculate the fuel required in still air and with the forecast 2.7-hour headwind, and make a go/no-go decision.
Read the worked answer
Still air: trip 2.2 × 38 = 83.6 L; taxi 4 L; reserve 0.75 × 38 = 28.5 L; total = 116.1 L. That is within 134.7 L, with 18.6 L margin.
2.7 h headwind: trip 2.7 × 38 = 102.6 L; taxi 4 L; reserve 28.5 L; total = 135.1 L. That exceeds the 134.7 L weight-limited maximum by 0.4 L. On paper it is not legal, and the margin is effectively zero.
Decision: do not depart with all four and 135 L. Either offload one 75 kg passenger (allowable fuel then becomes (1,157 − 985)/0.72 = 239 L, so full tanks are available), plan a fuel stop, or wait for the wind to ease. Do not treat a 0.4 L shortfall as acceptable; forecast wind and burn errors are larger than that.
3. If you depart with 135 L and the 2.7-hour headwind materialises, what is the fuel state at destination and is the reserve intact?
Read the worked answer
Fuel used = taxi 4 + trip 102.6 = 106.6 L. Remaining = 135 − 106.6 = 28.4 L. The 45-minute reserve at 38 L/h needs 28.5 L.
So the aircraft arrives with 0.1 L less than the fixed reserve - technically a breach - and with no fuel for a diversion to Whangarei (45 min = 28.5 L) or for a go-around at Claris. The correct in-flight decision, if the headwind is stronger than forecast, is to divert early or land at a planned fuel stop, not to press on and arrive with the reserve gone. The 45-minute reserve is a planning minimum, not a target.